https://doi.org/10.1130/G46738.1

Manuscript received 18 February 2019 Revised manuscript received 9 August 2019 Manuscript accepted 17 August 2019

© 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 23 September 2019

Initial Laramide tectonism recorded by Upper Cretaceous paleoseismites in the northern Bighorn Basin, USA: Field indicators of an applied end load stress W.T. Jackson Jr.1, M.P. McKay2, M.J. Bartholomew3, D.T. Allison1, D.L. Spurgeon2, B. Shaulis4, J.A. VanTongeren5 and J.B. Setera5 1Department of Earth Sciences, University of South Alabama, Mobile, Alabama 36688, USA 2Department of Geography, Geology and Planning, Missouri State University, Springfield, Missouri 65897, USA 3Department of Earth Sciences, The University of Memphis, Memphis, Tennessee 38152, USA 4Department of Geosciences, Trace Element and Radiogenic Isotope Laboratory (TRaIL), University of Arkansas, Fayetteville, Arkansas 72701, USA 5Department of Earth and Planetary Sciences, Rutgers University, New Brunswick, New Jersey 08901, USA

ABSTRACT and Liu, 2016), and plate-margin end load stress- Soft-sediment deformational structures associated with paleoseismicity (e.g., planar clastic es (e.g., Livaccari and Perry, 1993; Erslev, 1993; dikes) exist within Upper Cretaceous Mesaverde Group strata in the Laramide Elk Basin an- Tikoff and Maxson, 2001; Axen et al., 2018) ticline, northern Bighorn Basin (Wyoming, USA). Retrodeformation of the Elk Basin anticline as driving forces of Laramide tectonism. While to a horizontal Mesaverde Group position indicates that all basement offset is removed and models vary in methods and interpretations, flat- that clastic dikes exhibit a dominant northeast trend. The trend of clastic dikes corresponds to slab subduction of the Farallon plate is common- the interpreted northeast-southwest direction of early Laramide layer-parallel shortening, sug- ly required as a principal mechanism. Flattening gesting that the development of clastic dikes recorded initiation of basement deformation and of the Farallon plate beneath the North American Laramide tectonism. To determine the timing of clastic dike development, we present zircon U-Pb lithosphere commenced at ca. 90–85 Ma, pre- geochronology from the stratigraphically lowest sand-source bed generating upwardly injected sumably in response to the arrival and subduc- clastic dikes and a volcanic bentonite bed (Ardmore bentonite) above the stratigraphic interval tion of a buoyant oceanic plateau, which was containing clastic dikes. Weighted mean ages bracket clastic dike development between 82.4 and a conjugate feature to the Shatsky Rise in the 78.0 Ma. Our results imply initiation of basement deformation ∼8–15 m.y. prior than other esti- modern western Pacific Ocean (Saleeby, 2003; mates in the Bighorn Basin. Therefore, we interpret the development of clastic dikes in the Elk Liu et al., 2010). Numerical models by Axen Basin anticline to represent an initial phase of Laramide tectonism associated with an applied et al. (2018) show that an applied end load along end load stress transmitted from the southwestern North American plate margin in response to the plate margin associated with the collision of the collision of the conjugate Shatsky Rise oceanic plateau ca. 90–85 Ma. Results demonstrate the conjugate Shatsky Rise would have resulted how sedimentary responses in the foreland can be used to understand tectonic processes at plate in a compressional stress state in the overriding boundaries and provide spatial-temporal parameters for models of Laramide deformation. North American lithosphere, thereby promoting the development of Laramide tectonism. How- INTRODUCTION 1983; Stone, 1993; Hoy and Ridgway, 1997; ever, it remains unclear how the application of Late Cretaceous through Eocene Laramide- Cather, 2004; Tindall et al., 2010), basin subsid- an end load stress would be recorded in the rock style (basement-involved) deformation occurred ence (Mitrovica et al., 1989; Lawton, 1994; Heller record and, if so, how the end load stress would east of the Sevier thrust front in the western United et al., 2003; Leary et al., 2015), lulls in magmat- be distributed throughout the Laramide belt. States Cordillera (Fig. 1A; Dickinson and Sny- ic activity (Dickinson and Snyder, 1978; Hum- Soft-sediment deformational structures form der, 1978; DeCelles, 2004; English and John- phreys, 2009), deposition of synorogenic strata in response to natural processes such as rapid ston, 2004; Yonkee and Weil, 2015). Spatial and (DeCelles et al., 1991), exhumation of basement sedimentation and paleoseismicity (Obermeier, temporal observations of Laramide deformation arches (Omar et al., 1994; Crowley et al., 2002; 1996; Audemard and Michetti, 2011; Owen and continue to motivate geodynamic models aimed at Peyton et al., 2012), and paleoelevation estimates Moretti, 2011). Paleoseismites, defined as pre- understanding the driving forces and mechanisms (Fan and Carrapa, 2014; Fan et al., 2014). Neogene soft-sediment deformational structures for intraplate tectonism. The onset and duration To describe these spatial and temporal rela- associated with paleoseismicity, record syndepo- of Laramide deformation are temporally brack- tionships, models propose basal friction (e.g., sitional tectonism prior to lithification and the eted by the transition from marine to nonmarine Bird; 1998; Yonkee and Weil, 2015; Behr and onset of major orogenic events (Winslow, 1983; sedimentation (Dickinson et al., 1988; Raynolds, Smith, 2016; Copeland et al., 2017), hydrody- Bartholomew et al., 2002; Stewart et al., 2002; 2003; Cather, 2004), crosscutting structural and namic stresses and flow in the asthenosphere Bartholomew and Whitaker, 2010). Upper Cre- stratigraphic relationships (Wiltschko and Dorr, (e.g., Liu et al., 2008; Jones et al., 2011; Heller taceous through Eocene strata in the northern

CITATION: Jackson, W.T., et al., 2019, Initial Laramide tectonism recorded by Upper Cretaceous paleoseismites in the northern Bighorn Basin, USA: Field indica- tors of an applied end load stress: Geology, v. 47, p. 1059–1063, https://doi.org/10.1130/G46738.1

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/11/1059/4851992/1059.pdf by guest on 02 October 2021 MONTANA 108.9ºW A EBA SOUTH DAKOTA B A B Belt ELK BASIN 45ºN IDAHO BM ANTICLINE BWA BHB BH PRB A' N 012

N Thrust OC kilometers WR Laramide uplift WRB NEBRASKA LR MONTANA 45ºN FS 18EB03 Laramide basin GRB WYOMING GDB HB R 18EB01

Sevier Judith River UM PR Formation UTAH FR DB 40ºN UBUB WRP Claggett Formation PB 18EB03

Limit Clastic dike interval SRS SR Eagle UU Formation 18EB01 C DE MU COLORADO Telegraph Eastern CU SJU KP Creek KU RB Formation MAP EXPLANATION SJB Zircon U-Pb sample ARIZONA Cody Shale Outcrop with clastic dike(s) 0 400 SLU NEW MEXICO kilometers Fault A 110ºW 105ºW

Figure 1. (A) Spatial distribution of Laramide arches and basins throughout western United N States Cordillera, adapted from Heller and Liu (2016). Location of Elk Basin anticline (EBA) is F denoted in northern Bighorn Basin (Wyoming, USA). A-A′ line corresponds to section A-A′ in Figure 4. BH—Black Hills; BHB—Bighorn Basin; BM—Bighorn Mountains; BWA—Beartooth, Washakie, Absaroka Ranges; CA—Casper Arch; CU—Circle uplift; DB—Denver Basin; FR—Front Range; FS—Ferris, Green, Seminoe, Shirley Mountains; GDB—Great Divide Basin; GRB— Green River Basin; HB—Hanna Basin; HM—Hogback monocline; KP—Kaiparowits Plateau; KU—Kaibab uplift; LR—Laramie Range; MU—Monument upwarp; OC—Owl Creek Mountains; PB—Piceance Creek Basin; PR—Park Range; PRB—Powder River Basin; R—Rock Springs uplift; RB—Raton Basin; SJB—San Juan Basin; SJU—San Juan uplift; SLU—San Luis uplift; SR—Sawatch Range; SRS—San Rafael Swell; UB—Uinta Basin; UM—Uinta uplift; UU—Uncom- pahgre uplift; WR—Wind River Range; WRB—Wind River Basin; WRP—White River Plateau. (B) Geologic map and generalized stratigraphic column of Elk Basin anticline with distribution of clastic dikes (stars) and locations of zircon U-Pb samples (diamonds). Geologic map and stratigraphic column adapted from Jackson et al. (2016) and Engelder et al. (1997), respectively. Regional Laramide (LPS) Bighorn Basin (Wyoming, USA) contain paleo- Mesaverde Group strata are assigned Campan- seismites that are interpreted to record Laramide ian ages based on ammonite biostratigraphy (Gill deformation in the region (Bartholomew et al., et al., 1972), paleomagnetic analysis (Hicks et al., Clastic dikes (n=145) 2008; Stewart et al., 2008; Jackson et al., 2016). 1995), and the age of the Ardmore bentonite bed Thus, the objective of this study was to bracket (Hicks et al., 1999), a regionally correlatable unit Figure 2. (A–E) Examples of clastic dikes in the timing of paleoseismite development in the located stratigraphically in the middle part of the Elk Basin anticline (Wyoming, USA). Scale bar = 0.92 m. (F) Stereographic plot illustrating northern Bighorn Basin in order to evaluate the Mesaverde Group (Bertog et al., 2007). clastic dike measurements. Dots represent spatial-temporal evolution of Laramide deforma- poles to bedding with corresponding 2σ con- tion as it relates to an applied end load stress. We PALEOSEISMITES touring. Rose diagram demonstrates strike present field observations and structural analy- Soft-sediment deformational structures in of unfolded clastic dikes corresponding to sis of paleoseismites coupled with zircon U-Pb the form of clastic dikes, convolute bedding, and early Laramide layer-parallel shortening (LPS; gray shading) for Bighorn Basin and southern geochronology from Mesaverde Group strata in overturned subvertical vents (i.e., modern pipe Wyoming (e.g., Weil and Yonkee, 2012). the Elk Basin anticline, northern Bighorn Basin. features) are present within Mesaverde Group strata in the Elk Basin anticline (Bartholomew deformational structures (e.g., Obermeier et al., ELK BASIN ANTICLINE et al., 2008). We focused on clastic dike devel- 2002), all basement displacement is removed The Elk Basin anticline (Fig. 1B) is a north- opment for tectonic interpretations and to define (Jackson et al., 2016). The unfolded strike of west-southeast–trending fault-propagation fold the term paleoseismite because they provide the clastic dikes indicates a prominent northeast structure (McCabe, 1948; Engelder et al., 1997). most direct evidence of soft-sediment deforma- trend (Fig. 2F), which is compatible with inter- Beneath the anticline, the Elk Basin thrust fault tional structures associated with seismic shak- preted early Laramide layer-parallel shortening displaces Wyoming Province basement rock ing (e.g., Tuttle and Seeber, 1991; Obermeier, directions in the Bighorn Basin and central Wyo- ∼1900 m to the northeast, with net slip dissipat- 1996; Bourgeois and Johnson, 2001; Stewart ming (e.g., Weil and Yonkee, 2012). Together, ing toward the surface (Stone, 1993). The Elk Ba- et al., 2002). In the Elk Basin anticline, the Eagle these observations suggest that the clastic dikes sin anticline is erosionally breached, exposing an Formation contains 71 outcrops with 145 clas- (paleoseismites) recorded seismic shaking as- ∼250-m-thick sequence of Mesaverde Group stra- tic dike segments. Clastic dikes exhibit planar sociated with initial displacement of basement ta (Fig. 1B). Mesaverde Group strata represent a shapes (Figs. 2A–2C), tend to taper upward rock beneath the Elk Basin anticline. progradational sequence of marine, shallow-ma- and/or terminate at the base of the overlying rine, and nonmarine deposits along the western sandstone (Fig. 2D), and contain en echelon GEOCHRONOLOGY margin of the Cretaceous Interior Seaway (Swift segments (Fig. 2E), indicating that they were We dated the stratigraphically lowest sand- and Rice, 1984; Fitzsimmons and Johnson, 2000; injected along preexisting, mixed-mode (open- source bed in the Mesaverde Group (Eagle For- Swift et al., 2008). At the Elk Basin anticline, ing and shear) fracture avenues (Jackson et al., mation) that produced upwardly injected clastic the Mesaverde Group is subdivided from old- 2016). When the Elk Basin thrust fault is retro- dikes (sample 18EB01) and a volcanic bentonite est to youngest into the Telegraph Creek, Eagle, deformed to a horizontal Mesaverde Group, a deposit (sample 18EB03) in the Claggett Forma- Claggett, and Judith River Formations (Fig. 1B). requirement for the generation of soft-sediment tion (Fig. 1B). Zircon grains were separated and

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/11/1059/4851992/1059.pdf by guest on 02 October 2021 B 81.3 ± 3.3 Ma 18EB03 Obradovich (1993) and Hicks et al. (1995) dat- tion and exhumation were products of propagat- MSWD = 1.5 Ardmore ed the Ardmore bentonite bed in the Elk Basin ing stress associated with enhanced intraplate bentonite bed anticline using Ar/Ar to 80.54 ± 0.55 Ma and coupling over the flat-slab region. We suggest 80.71 ± 0.55 Ma, respectively. Our weighted a connection exists between the development of mean average is within error of all three age de- paleoseismites in the Elk Basin anticline and the terminations for the Ardmore bentonite. Because collision of the conjugate Shatsky Rise along the Bracketed Timing of Paleoseismite Development the clastic dikes originated from the sand-source southwestern North American plate margin ca. A 18EB01 clastic dike bed, we interpret the weighted mean average for 90–85 Ma (e.g., Saleeby, 2003; Liu et al., 2010). 81.0 ± 1.4 Ma sand-source bed sample 18EB01 as the lower age bound for paleo- Collision of the conjugate Shatsky Rise with MSWD = 0.8 seismite development. Both of our detrital U-Pb the North American plate margin established a age determinations correspond to regional ammo- compressional stress through the upper-mantle nite zones (Baculites obtusus) and paleomagnetic lithosphere (Axen et al., 2018). Weil et al. (2014) 70 75 80 85 90 95 100 105 110 Zircon U-Pb Age (Ma) results for the Mesaverde Group in the northern suggested that stress propagation through the Bighorn Basin (Hicks et al., 1995). The overlap in North American plate was enhanced because of Figure 3. (A,B) Weighted mean averages of youngest zircon U-Pb age population for sample our ages and previously reported age determina- a relatively cold and thick lithosphere, while also 18EB01 (A) and sample 18EB03 (B) in Elk Basin tions suggests that Mesaverde Group strata were noting that heterogeneities in the upper lithosphere anticline, northern Bighorn Basin (Wyoming, rapidly deposited in the northern Bighorn Basin often collect propagating stresses, resulting in pre- USA). Samples are ordered relative to strati- region. This interpretation fits well with Late Cre- ferred zones of deformation. Bader (2018) sum- graphic position. Horizontal bars represent 2σ error for individual analyses. Green bars indi- taceous subsidence patterns for the Bighorn Basin marized how tectonically inherited basement an- cate grains that were included in weighted mean (e.g., May et al., 2013). Our results indicate that isotropies in the Wyoming Province controlled age calculation. Dashed lines represent kernel the clastic dikes developed ca. 81 Ma; however, the spatial distribution of Laramide deformation density estimations for each sample at a band- to provide an encompassing interpretation of the in the Bighorn Basin region. Thus, we envision width of 15. Bracketed timing of paleoseismite data, we use the minimum 2 age uncertainty an applied end load stress, established at the plate development from 82.4 to 78.0 Ma was estab- σ lished by using minimum 2σ age for 18EB03 and for sample 18EB03 (Ardmore bentonite) and the margin, propagating through the overriding North maximum 2σ age for sample 18EB01. MSWD— maximum 2σ age uncertainty for sample 18EB01 American plate and coalescing at preexisting mean square of weighted deviates. (sand-source bed) to bracket the development of basement weaknesses, culminating in basement- clastic dikes from 82.4 to 78.0 Ma (Fig. 3). involved thrusting. Displacement along basement mounted using standard mineral extraction meth- rock then generated earthquake waves that ap- ods at Missouri State University (Springfield, Mis- DISCUSSION plied a localized shear stress to unconsolidated, souri, USA), and U-Pb analyses were conducted Our interpretation of 82.4–78.0 Ma for initial ­saturated sedimentary layers at or near the surface. by laser ablation–inductively coupled plasma– Laramide tectonism in the Elk Basin anticline The applied shear stress caused an increase in the mass spectrometry (LA-ICP-MS) at the University is ∼8–15 m.y. older than other estimates for the pore-fluid pressure for the saturated sand-source of Arkansas (Fayetteville, Arkansas) and Rutgers Bighorn Basin (Peyton et al., 2012; Fan and layer, which overcame overburden pressures and University (New Brunswick, New Jersey) (Table Carrapa, 2014; Stevens et al., 2016; Beaudoin produced soft-sediment deformational structures DR2 in the GSA Data Repository1). Analyses that et al., 2018). However, our results do correlate (e.g., clastic dikes). As the conjugate Shatsky Rise yielded discordant results (>30% discordant or to thermochronology results from the Beartooth subducted into and past the trench, the Farallon 10% reverse discordant) or high U (>2000 ppm) Range, which bounds the Bighorn Basin to the plate transitioned to flat-slab subduction, result- were excluded. Reported ages for each sample are west (Carrapa et al., 2019). Carrapa et al. (2019) ing in subsequent, more traditionally interpreted based on a weighted mean average of the young- suggested that early regional Laramide deforma- Laramide deformation (Fig. 4). est age population, which we defined as all grains within 2σ uncertainty of the youngest grain. Eight AA' Figure 4. Plot illustrat- grains from sample 18EB01 were used to deter- SW estimation for the easternmost portion of NE ing spatial and temporal the Farallon plate with respect to North America relationship between east- mine the weighted mean average of 81.0 ± 1.4 Ma ernmost edge of Farallon (Fig. 3). Four grains from sample 18EB03 were Elk Basin anticline 100 plate (gray line) and south- used to determine the weighted mean average of APPLIED END LOAD STRESSPaleoseismites western North America 81.3 ± 3.3 Ma (Fig. 3). plate margin (adapted from Age (Ma ) Because the bentonite bed represents a vol- Copeland et al., 2017). This relationship incorporates canic deposit, we interpreted the weighted mean magmatism (solid gray average for sample 18EB03 as an approxima- Collision of the dots), modeled location of tion of the true depositional age of that bed. The conjugate Shatsky Rise conjugate Shatsky Rise (tri- stratigraphic position of the bentonite bed above angles), youngest marine deposits (diamonds), the clastic dike interval in the Mesaverde Group 40 timing of initiation of establishes the upper age bound for paleoseis- Initiation of Laramide Laramide deformation (cir- mite development in the Elk Basin anticline. This deformation in the Bighorn Basin cles), timing of cessation bentonite bed, regionally referred to as the Ard- of Laramide deformation 0 500 1000 1500 2000 (vertical bars), and attain- more bentonite, is 80.04 ± 0.40 Ma based on Ar/ Distance (km) ment of maximum surface Ar dating of biotite grains (Hicks et al., 1999). elevation (hexagons). We correlated arrival and collision of conjugate Shatsky Rise to generation of paleoseismites (clas- tic dikes) in Elk Basin anticline, northern Bighorn Basin. Multi-million-year lag time from collision 1GSA Data Repository item 2019369, Table DR1 of conjugate Shatsky Rise to development paleoseismites in Elk Basin anticline (red dashed line) (clastic dike field data) and Table DR2 (U-Pb geochro- represents an opportunity to quantify relationships between sedimentary responses in foreland nology), is available online at http://www.geosociety. and tectonic processes at plate margin during Laramide tectonism. Because paleoseismites org/datarepository/2019/, or on request from editing@ indicate Laramide tectonism 8–15 m.y. prior to estimations for Bighorn Basin (green box), we pro- geosociety.org. pose that paleoseismites are surficial features that record initial phase of basement deformation.

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Society of America Special Papers, v. 280, p. 125– deformation as well as the temporal relationships Bird, P., 1998, Kinematic history of the Laramide 146, https://doi.org/10.1130/SPE280-p125​ . orogeny in latitudes 35–49N, western United Fan, M., and Carrapa, B., 2014, Late Cretaceous–early between tectonic processes at the plate margin and States: Tectonics, v. 17, p. 780–801, https://doi​ Eocene Laramide uplift, exhumation, and basin sedimentary responses in the foreland. .org/10.1029/98TC02698. subsidence in Wyoming: Crustal responses to flat Bourgeois, J., and Johnson, S.Y., 2001, Geolog- slab subduction: Tectonics, v. 33, p. 509–529, ACKNOWLEDGMENTS ic evidence of earthquakes at the Snohornish https://doi​.org/10.1002/2012TC003221. The Geological Society of America’s Stephen E. Delta, Washington, in the past 1200 yr: Geo- Fan, M., Heller, P.L., Allen, S.D., and Hought, B.G., Laubach Award and internal faculty awards from logical Society of America Bulletin, v. 113, 2014, Middle Cenozoic uplift and concomitant the University of South Alabama and Missouri State p. 482–494, https://doi​.org/10.1130/0016- drying in the central Rocky Mountains and ad- University supported this study. Previous discussions 7606(2001)113<0482:GEOEAT>2.0.CO;2. jacent Great Plains: Geology, v. 42, p. 547–550, with William R. Dupré and Kevin G. Stewart aided Carrapa, B., DeCelles, P.G., and Romero, M., https://doi​.org/10.1130/G35444.1. in our paleoseismite interpretations. This work ben- 2019, Early inception of the Laramide orog- Fitzsimmons, R.J., and Johnson, S.D., 2000, Forced efited from comments by Theresa M. Schwartz, Gary eny in southwestern Montana and northern regressions: Recognition, architecture and gen- J. Axen, and two anonymous reviewers. 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